Question: William is 4 times as old as Gabriela and is also 15 years older than Gabriela. How old is William?
Explanation: We can use the given information to write down two equations that describe the ages of William and Gabriela. Let William's current age be $w$ and Gabriela's current age be $g$ $w = 4g$ $w = g + 15$ Now we have two independent equations, and we can solve for our two unknowns. One way to solve for $w$ is to solve the second equation for $g$ and substitute that value into the first equation. Solving our second equation for $g$ , we get: $g = w - 15$ . Substituting this into our first equation, we get the equation: $w = 4$ $(w - 15)$ which combines the information about $w$ from both of our original equations. Simplifying the right side of this equation, we get: $w = 4w - 60$ Solving for $w$ , we get: $3 w = 60$ $w = 20$.